2.
Section 2: Assessment of organ function for drug dose adjustment
Stuart Goldstein & Frieder Keller
The organ probably most impacting drug kinetics is the kidney. Fortunatley, there is a close
correlation between the function of the glomeruli and of the tubuli. Impaired tubule function by for
example acute kidney injury and tubule necrosis will be accompanied by impaired glomerular
filtration. Impaired glomerular function by for example rapidly progressive glomerulonpehritis will
impact tubular secreation, metabolism and reabsorption of drugs in parallel. Thus, the glomerular
filtration rate (GFR) has been established as a global measure of kidney function.
Selective tubular alterations are rare with some exceptions. For example the tubular creatinine
secretion is decreased by cimetidine or co-trimoxazole. The tubular secretion of anionic drugs such as
penicillin or cidofovir is inhibited by probenecid. Known inhibitors of the organic cation transporters
are cimetidine, ranitidine and tyramine. Except for such drugs, the tubule function can be subsumized
under the GFR.
Several methods are available to estimate the glomerular filtration rate (GFR) as a measure of the
kidney function in the individual patient. (1) The endogenous creatinine clearance is considered
anachronistic due to errors with the urine collection and due to the 24 hours delay in the measurments
results.1 (2) The inulin clearance is propagated as golden standard but rarely performed except for
research purposes. (3) Radiocontrast agents like iothalamate are no ideal standard for GFR
measurements since an extrapolation of the half-life or of the area under the curve AUC is needed.
Such extrapolation is the less complete and less reliable the more the kidney function is impaired and
the more prolonged the half-life is.2 (4) The isotope GFR measurments are not practical for individual
drug dose calculations since they are expensive and no everywhere and everytime available. (5) The
most practical and valuable measures of the GFR are the urine-freee methods based on serum
creatinine and specific other variables. The different urine-free methods to estimate the GFR have
different advantages and disadvantages that we should like to discuss here.
GFR Estimates
For drug dose adjustment the kidney function should be estimated by the GFR. With all of the
available methods the independent variable, the GFR is estimated from the dependent but measurable
variable the serum creatinine (Screa). To convert the serum creatinine from mg/dl to mcmol/l
multiplication by the factor of 88.4 is needed (SCrea[mcmol/L] = 88.4 x SCrea[mg/dL]).
It is important to note that in the last years the Jaffe method to estimate the serum creatinine has been
replaced by the new calibrated isotope dilution mass spectrometry technique (IDMS) yielding serum
creatinine estimates that are 5% lower on average than previously. This must be considered when
using equations that have been established based on the creatinine measured by the Jaffe method
(SCrea[IDMS] = 0.95 x SCrea[Jaffe]).
The most frequently used urine free method is the Cockcroft and Gault equation (C&G).3 This
equation primarily estimated the creatinine clearance from serum creatinine and age and is used now
as an GFR estimate.
140 − Age( years)
GFR = ⋅ Weight (kg ) ⋅ 0.85( female)
0.82 ⋅ Screa( µmol / L)
The frequently used C&G equation has been validated recently for the new calibrated creatinine. This
new equation does not have a coefficient and is more easily to memorize and to use for calculations
than other GFR estimates.4 It is an advantage of the coefficient-free C&G equation for individual drug
dose adjustment that the body weight is considered. Treating overweight patients, however, the lean
body weight should be used.
155 − Age( years)
GFR = ⋅ Weight (kg ) ⋅ 0.85( female)
Screa( µmol / L)

3.
For an automatic implementation in the chemical laboratory outprint, the C&G equation is not suitable
since the body weight is usually not available automatically. This is the big advantage of the
modification of diet in renal disease (MDRD) equation that it is standardized to a body surface area of
1.73 sqm. The MDRD GFR is using the serum creatinine in mg/dl.5
GFR = 186.3 ⋅ Screa −1.154 ⋅ Age −0.2031.212[black ] ⋅ 0.742[ female]
The MDRD equation can be used for individual drug dose adjustment when the body surface area
(BSA) is considered (GFR = MDRD x BSA / 1.73m2).6 This very popular MDRD equation has been
transformed also for the new IDMS calibrated creatinine measurements since it holds (186.3 x (1 /
0.95)^-1.154 = 175.6).7
GFR = 175.6 ⋅ Screa −1.154 ⋅ Age −0.2031.212[black ] ⋅ 0.742[ female]
The primary MDRD equation has been validated only for a GFR < 60 ml/min. Therefore, the new
CKD EPI equation has been proposed to cover the full range of the chronic kidney disease spectrum.8
GFR = 141 ⋅ min( Screa / κ ,1)α ⋅ max(Screa / κ ,1) −1.209 ⋅ 0.993 Age ⋅ 1.159[black ] ⋅ 1.018[ female]
In the CKD-EPI formula expressed as a single equation the Scr is serum creatinine (mg/dL), κ is 0.7
for females and 0.9 for males, α is -0.329 for females and -0.411 for males, min indicates the
minimum of Scr/κ or 1, and max indicates the maximum of Scr/κ or 1 and Age is measured in years.
Independent from age, weight, sex, race and muscle mass is the GFR estimate derived from serum
cystatin C (nephelometric CystC mg/L) that most simply can be calculated by the Oerebro formula.9
100
GFR _ CystatinC = − 14
CystC (mg / L)
It depends on the local experience and availability of data which one of the above equations should be
used for assessment of the individual GFR to adjust the drug dose. We think it is more important to
have any GFR estimate and to think about dose adjustment than to have a most exact estimate but to
not have it in time and without big effort.
The GFR estimates holds for chronic kidney disease. But also in acute kidney injury a measure of
individual kidney function is needed to adjust the drug dose. Such a measure can be derived from the
GFR equations by taking into account that such estimates are always delayed.10 In patients with
oliguric or anuric kidney failure the GFR is zero.
Liver, Physical Function and Age
In contrast to the kidney function, the liver function cannot be easily quantitated and assessed for drug
dose adjustment. The Child-Pugh score lacks the sensitivity to quantitate the specific ability of the
liver to metabolize individual drugs.11 The proposal has been made to use the new MELD score to
assess the liver function. Liver failure is considered when the MELD score is 20 or above where the
specification is (etiology: 0 if cholestatic or alcoholic, 1 otherwise). However, the MELD score is
determined also by the serum creatinine and thus by the kidney function.
MELD = +3.8 ⋅ ln[bilirubin(mg / dL)] + 11.2 ⋅ ln[ INR ] + 9.6 ⋅ ln[SCrea(mg / dL)] + 6.4 ⋅ [etiol ]
Another determinant of drug kinetics and dynamics might be seen when the physical function is
impaired. Physical function should be assessed. The Karnofsky performance score (70% weakness,
40% bed-bound) or the frailty index have been proposed to quantitate such co-morbidity and the
effects of age. But no systematic correlation to drug kinetics and dynamics has been performed yet.
In elderly people there is a loss of kidney function.. This should be assessed by the individual GFR.
The effect of age on body composition or liver function is less impressive. We have found age related
changes to be most impacting the drug half-life (1.39-fold ± 61%).12 Contrasting to common
expectations drug clearance and volume parameters have been found to be less influenced by aging
processes (Tab. 1).

5.
Section 3: Calculating drug doses in CKD
Darren Grabe and Lesley Stevens
Introduction
Calculation of drug doses in patients with kidney disease is complex.1-3 The challenge is to
integrate the pharmacokinetic profile of a drug with an accurate estimate of the patient’s
kidney function in the context of the specific clinical scenario including potential for adverse
events relative to too high, or too low, a dose.4.5 Despite numerous published guidelines6-13
regarding drug dosing, we have not yet succeeded in determining best methods to calculate
drug dosages for particular patients as evidenced by the high rate of adverse events and drug
related morbidity and mortality.14-16
Currently, adjustment of drug dosages is performed based primarily upon the package inserts.
In most cases, the pharmacokinetic studies were conducted when the drug or the relevant
metabolites are cleared by the kidney and have a narrow therapeutic window.17.18 In that
regard, the glomerular filtration rate (GFR) or its estimate is assumed to capture all aspects of
the effect of kidney disease on the pharmacokinetic profile of a drug under the “intact nephron
hypothesis.”19 The problem arises is that both dosing guidelines as well as clinicians’
application of these guidelines to individual patients, do not explicitly incorporate other
factors that while may not correlate with GFR (e.g. decreased absorption or
hypoalbuminemia), may be present in patients with kidney disease.5,20-22
Impact of Kidney Disease on Pharmacokinetics
Pharmacokinetic parameters The most apparent alteration of CKD on drug pharmacokinetics
is the reduced clearance of the drug
Table 1: Impact of CKD on Pharmacokinetics
or its metabolite due to a reduction
in GFR with subsequent Pharmacokinetics Kidney Disease Incorporated
Parameters Effects into Drug Dosage
accumulation of drug or relevant
metabolites. However, kidney Absorption + N
Intestinal and first + N
disease is associated with a number
pass metabolism
of other factors which may influence Distribution ++ N
drug dosing and include changes in Clearance
absorption5,22, first-pass metabolism, Renal +++ Y
23
hepatic metabolism , drug Nonrenal ++ N
20 24
transport , protein binding and
volume of distribution25 (Table 1). The impact of these other factors on drug dosing is unclear
and deserves attention and further study.
Assessment of Kidney Function For most drugs, dosing guidelines were developed prior to
standardized calibration of creatinine assays and reporting estimated GFR calculated using the
Modification of Diet in Renal Disease (MDRD) Study equation (eGFR). The discussion as to
which estimate can be used to adjust drug dosages, often centers around which equation,
however, it is the issue of the creatinine assay that is most relevant.
Historically, there has been substantial variability in serum creatinine values reported by
different clinical laboratory creatinine methods28. Pharmacokinetic studies performed using
non-standardized creatinine methods obtained results that were dependent upon the particular
creatinine method used, leading to substantial variation when translated into practice29. Use

6.
of standardized creatinine methods will lead to less variation in estimating kidney function
and more consistent drug dosing. However, for drugs for which the studies have already been
performed, the kidney function cutoffs, regardless of which equation was used, are likely to
be substantially different from those identified in the original study. Using standardized
creatinine, the two most commonly used equations, the Cockcroft and Gault formula and the
MDRD Study equation, result in the same drug dosage for most patients30. It is important to
remember that for all creatinine based equations, the limitations of serum creatinine must be
considered31.
Determination of Drug Dosing Guidelines
Loading Dose Loading doses may be required if a drug has a long half life and is used in
situations where there is a need to achieve steady state rapidly. Equation (1) describes the
dependence of the half-life on the volume of distribution and the clearance of the drug.
Changes in either of those factors can result in an increase or decrease in the half-life. It takes
approximately 3.3 half-lives to achieve steady state drug concentrations
t1/2 = [(0.693) (Vd)]/(Cl) Equation 1
Equation (2) describes how the loading disease is related to the desired initial concentration of
the drug) and the volume of distribution.
Loading dose = (Cinitial) (Vd) Equation 2
Equation 3 indicates how a modified loading dose can be calculated given known changes in
volume of distribution.
Usual loading dose = Normal Vd Equation 3
Modified loading dose = patient’s Vd
Most published guidelines do not include adjustment in loading dose, despite the well
documented changes in volume of distribution that occur in patients with kidney disease.
Maintenance dose The goal of a maintenance dosing regimen is to preserve the steady-state
drug concentrations as would occur if the patient had normal kidney function. A change in
clearance requires a change in dose to maintain drug concentration (equation 3).
Maintenance dose =(Caverage) (Cl) Equation 3
The strategies that are employed to accomplish this goal include reducing the rate of a
continuous infusion, reducing the dose, prolonging the interval between doses, or a
combination of these strategies. In general, reducing the dose but maintaining the same
frequency of dosing will result in more stable drug levels. However, that may not be
appropriate for toxic medications, where achieving low trough levels is the goal. For
example, aminoglycoside regimen generally use the strategy of extending the interval
between doses.
Bioavailability is another important factor to consider when medications are to be
administered orally. However, the impact of impaired kidney function on bioavailability of

7.
most drugs is unknown, and may be determinant on fluctuating individual factors such as
concomitant drugs or gastric pH, making it difficult to establish a consistent drug regimen.
The final consideration is the effect of kidney replacement therapy. Depending on the
modality, dialysis may remove drugs and relevant metabolites either intermittently or
continuously. The total clearance of drugs is dependent on residual kidney function, drug and
metabolite properties, dialysis properties (e.g. duration) and membrane properties (e.g. pore
size). All must be considered in establishment of the maintenance dose.
Determination of Individualized Patient Dose
Calculating drug dosages for individual patients Calculating drug dosage in patients with
kidney disease requires consideration of the impact of impaired kidney function on
pharmacokinetic parameters. In particular, application of the drug dosing guidelines to an
individual patient should be sensitive to the pharmacodynamics of the individual drug that
may not be incorporated into the dosing guideline (eg decreased absorption due to use of
chelating medications) as well as patient related factors including urgency of the clinical
situation, co-morbid conditions, and costs (Table 2). Thus, initial assessment of the patient is
critical to obtain Table 2: Determination of Individualized Patient Dose
relevant information to
Factor Method of Ascertainment Modify
choose a proper drug,
dose and schedule. This Clearance eGFR/eCrCl Maintenance
Volume of distribution Level of drug after initial dose Loading
initial assessment Urgency of clinical situation Clinical judgment Loading
should include a Impact of fluctuations in Clinical judgment Maintenance
history, physical steady state levels
examination including Patient’s financial situation Clinical judgment Maintenance
weight and an Medication interactions Profile review Loading,
maintenance
evaluation of volume
status, and medication history to identify possible drug interactions. Laboratory data is
required to assess kidney function as well as other organ function.
Measurement of Therapeutic Drug Levels For all drugs, the drug dosing guidelines are only
appropriate if the conditions are stable. Changes in any of the factors mentioned will require
further modifications to individual patient’s regimens. Measuring drug concentration may be
one way to optimize therapeutic regimens and account for changes in individuals over time.
However, therapeutic drug monitoring requires availability of a reliable assay and known
correlation of drug concentration to therapeutic and toxic. In addition, hypoalbuminemia may
influence interpretation of drug levels as the total drug may be reduced but the active drug
may not be reduced. Active drug cannot be measured and therefore clinicians must consider
the impact of hypoalbuminemia in interpretation of measured drug concentrations.20-22
Conclusion
Calculation of drug doses in patients with CKD is complex and often ignores other relevant
factors which may influence outcomes. Changes in the drug clearance associated with
reductions in the GFR are expected. However, data outlining the influence of CKD on
absorption, transport, non-renal clearance, protein binding and distribution are limited.

10.
Section 4: Calculating drug doses in AKI
Brian Decker and Deborah Pasko
Calculating drug doses in AKI: An Overview
Deborah Pasko
Introduction
Over the past several years the assessment of kidney function has been through many revisions. First
the definition of acute versus chronic kidney failure was established, and then there was a transition
from acute renal failure to acute kidney injury. Now within the past couple of years has been the
introduction of AKI biomarkers, RIFLE criteria, and the ongoing discussion about Cockcroft-Gault vs
MDRD. Despite all of the changes in nomenclature and definitions AKI still affects many patient
populations, but the intensive care patient seems to be most vulnerable. The incidence of AKI for
intensive care patients, including pediatric patients, can range from 1-25% with mortality rates of
15-60% [1,2,3]. The high mortality rate of AKI can be attributed to multiple physiological changes
that occur in these patients. AKI is a constellation of symptoms and can start as minor physiological
changes and may manifest into acute failure requiring renal replacement therapy. More often than not
patients experiencing any stage of AKI are also receiving medications, some life-sustaining. It is
important for clinicians to understand how to manage pharmacotherapy regimens during each stage of
AKI in addition to applying pharmacokinetic and pharmacodynamic principles so medication
regimens can be optimized and kidney function can be protected.
Applying pharmacokinetic principles in AKI
The application of basic and complex pharmacokinetic principles involving changes in absorption,
distribution, metabolism, and excretion is the first step to optimizing drug therapies for patients with
AKI. Critically ill patients typically have minimal oral intake of food and liquids and rely upon
intravenous fluids and for fluid maintenance and nutrition. In addition H2-antagonists and proton
pump inhibitors are used for stress ulcer prophylaxis and significantly alter the gut pH to either neutral
to slightly basic. Any orally administered drug needing an acidic environment may not be readily
absorbed and may not be effective. Other absorption alterations may include slow GI motility,
prolonged intestinal transit times, bacterial colonization, and necrotizing enterocolitis (seen in
neonates). Intravenous administration of drugs for patients with AKI may need to be considered to
assure appropriate absorption.
Drug distribution is one of the most important, but yet the most complicated, pharmacokinetic
principle to understand for patients with AKI. There is a fine balance between adequate hydration for
kidney perfusion and total body fluid overload. There have been numerous studies conducted about
the impact of fluid overload on patients with AKI, both adult and pediatric [4,5,6,7,8]. These studies
have concluded that critically ill patients should initially be managed in a slightly negative fluid
balance after initial adequate fluid resuscitation has been achieved [9].This can be achieved by three
strategies: 1) fluid restriction, 2)diuretic therapy, and 3) renal replacement therapy. Fluid restriction in
critically ill patients is often difficult because of the needed fluid to deliver drugs and nutrition.
Diuretic therapy has been the mainstay of therapy for patients initially, however studies now suggest
once patients reach a fluid overload status >5% renal replacement therapy should be considered, and
that continual use of diuretics may have a negative impact on patients [10,11]. Any combination of
these three fluid management strategies may cause rapid or constant fluid removal and will
significantly affect the distribution of drugs. For drugs that normally have small volumes of
distribution (Vd) any increase or decrease in fluid volume in the patient will cause a substantial dose
adjustment for that drug. An example of this is aminoglycosides which normally have a Vd of
approximately 0.2-0.35 L/kg in a euvolemic patient and is a peak/mic-dependent drug. If initial serum
concentrations are, for example, 8 mg/L for a peak concentration, and 1 mg/L for a trough with a Vd
of 0.25 L/kg, then doubling of the Vd to 0.5L/kg will result in a peak of 4 mg/L and trough of 0.5 mg/
L. At this point the drug has a decreased area under the curve for the peak serum concentration and
will have approximately half of the killing effect on bacteria. In contrast if the fluid overloaded patient
has a peak of 8 mg/L and a Vd of 0.5L/kg and the fluid is removed and the Vd decreases to 0.25L/kg
then the peak serum concentration is now 16 mg/L, and the trough concentration would be 2 mg/L.

11.
Fluid shifts such as these can happen within minutes to hours for patients with AKI. Constant re-
evaluation and monitoring of the drug therapies, including frequent assessment of serum
concentrations is warranted. Drugs that have larger volume of distributions are less affected by fluid
shifts and require less monitoring unless they have a very narrow therapeutic window. Moreover,
another key component of drug distribution is protein binding. Patients with AKI can experience
protein losses and/or not enough protein intake, and alterations in protein binding when in a uremic
state. In addition any administration of blood and/or albumin products will also affect the drug protein
binding. Again, frequent monitoring is needed, especially when using highly protein bound
anticonvulsants, such as phenytoin. The next pharmacokinetic category is metabolism and will be
explained in much greater detail in the second half of the manuscript.
The final pharmacokinetic parameter, and probably the most studied in patients with AKI, is excretion,
or elimination. Despite multiple studies however, there is still controversy about what equation most
accurately determines glomerular filtration rate (GFR) in patients with AKI [12,13]. The options
include the standard Cockcroft–Gault, Jeliffe, or modified Jeliffe, and more recently the Modification
of Diet in Renal Disease (MDRD) [12]. The gold standard equation for pediatric patients is still the
Schwartz equation. Historically, all drug studies have been based on assessing creatinine clearance or
GFR [14]. There is now emerging discussions however about using MDRD as the equation to estimate
kidney function for drug dosing [15]. Most of these equations involve the use of creatinine, which is
known to be a late marker in AKI. Instead, maybe drug dosing for patients with AKI needs to take a
different approach. There are now more sensitive early biomarkers, such as NGAL, cystatin C,
interleukin 18, and kim-1 [16], that could be used in conjunction with the RIFLE [17] and pRIFLE
[18] criteria to better predict when to make adjustments in drug therapy regimens. There is also a
better understanding of the pathogenesis of progressive kidney disease [19]. These three approaches
could be used in combination to proactively suggest changes in drug doses and/or intervals early on,
during the risk phase instead of waiting until injury occurs and then retroactively making changes to
the drug regimens. For example if the patient enters the first stage of AKI assessed by a drop in urine
output and biomarkers are elevated then the medication list should be reviewed for any nephrotoxic
medications that should either be discontinued, changed to another therapy, or be dose adjusted.
Examples would include holding, decreasing dose, or extending intervals of drugs including
aminoglycosides, vancomycin, teicoplanin, methotrexate, antivirals (acyclovir, ganciclovir, foscarnet)
until further assessment of renal function could be made. This approach may aid in protecting, or
preserving current renal function. This approach, however, has never been studied and would need
further clinical trials to determine the usefulness of such an approach.
Clinical pearls and recommendations
Drug dosing in AKI differs from CKD from several perspectives. The most important points surround
a balance of knowing when to give more of a drug based on Vd and/or clearance in comparison to
giving less. The previous aminoglyoside example demonstrates how important increasing the dose
based on the current Vd, despite if the patient is having worsening renal function. Doses may need to
be increased but intervals further extended to prevent accumulation. For patients with AKI less is not
always better. This is especially true when dosing antibiotics in patients with AKI. Calculating drug
doses in patients with AKI may seem complicated. Principles to use include the use of standard
pharmacokinetic equations, therapeutic drug monitoring (when available) and frequent assessment of
fluid status and renal function of the patient. In addition for drugs requiring continuous administration
(pressors, opioids, etc) and titration these agents should just be titrated base on desired physiological
outcomes and should not be adjusted based on fluid status, unless significant fluid changes have
occurred.
Another component that clinicians typically forget or fully do not understand is the non-renal
clearance of drugs. This aspect of drug metabolism and elimination is significantly altered in AKI and
physiological shifts can occur that may also necessitate higher dosing of certain agents.
Understanding non-renal clearance can also aid clinicians to optimize drug therapies for patients with
AKI.

13.
Section 4: Calculating drug doses in AKI (continued)
Calculating drug doses in AKI: Impact of non-renal clearance
Brian Decker
Introduction
It has been well established that the attenuation of the critical clearance processes of glomerular
filtration and tubular secretion observed in acute kidney injury (AKI) and chronic kidney disease
(CKD) hinder the excretion of renally-eliminated drugs. For CKD the literature that describes these
processes and the disposition of renally-eliminated drugs is robust leading to the development of
specific drug dosing algorithms to guide pharmacotherapy in patients with CKD and ESRD patients
receiving hemodialysis. However, this is not true for AKI; clinically reliable drug dosing algorithms
for AKI have remained elusive. This is likely due to the quite variable magnitude and physiological
milieu of AKI which makes the research and prediction of drug disposition exceedingly more difficult.
In addition, the underlying paucity of adequate tools to accurately determine renal function in the
acute setting is a contributing factor. Moreover, this pharmacotherapeutic approach of focusing solely
on renally-eliminated medications in renal failure patients is incomplete because it neglects non-renal
clearance. This is clinically significant because emerging in-vitro animal and human studies have
shown that nonrenal clearance, dominated by hepatic CYP450-mediated metabolism, is diminished by
the toxic, uremic environment found in renal failure. Administering the usual, full dose of a
metabolized medication to a renal failure patient may involve a similar risk for drug toxicity as
administering the full dose of a renally-eliminated medication. However, drug toxicity is not the only
concern as it is conceivable that the metabolic transformation of pro-drugs to their active forms could
also be inhibited by uremic solutes. Complicating matters further, the fluctuating concentrations of
uremic solutes in a patient receiving hemodialysis likely results in periods of both high and low drug
metabolism making predictions of drug action and disposition even more difficult. With notable
exceptions, the majority of the current research investigating non-renal clearance in AKI and CKD
utilizes in-vitro and animal models. Within the AKI literature there appears to be disagreement with
the majority of animal studies showing no effect on CYP450-mediated metabolism from AKI and the
human studies demonstrating decrements in nonrenal clearance from AKI. The purpose of this
summary is to describe what is known regarding AKI and non-renal clearance, to provide
recommendations for dosing metabolized medications in AKI and to outline potential avenues for
further research.
AKI and CYP450- mediated metabolism
The literature describing non-renal clearance in CKD has repeatedly shown that uremic solutes have
an inhibitory effect on CYP450-mediated metabolism. Multiple animal and the few human studies
have demonstrated a reduction in the transcription and/or metabolic activity of hepatic and intestinal
CYP450 enzymes by uremic solutes in CKD [1-15, 46-48]. This contrasts with the literature
describing the impact of AKI on non-renal clearance which surprisingly shows that in the majority of
animal studies no effect on CYP450- mediated metabolism from AKI. Of the current 13 studies that
examined the effect of AKI on hepatic drug metabolism using various medication probes, nine studies
did not demonstrate any impact on hepatic metabolic activity [16-18, 21-26, 45]. The remaining four
studies varied showing both an increase and a decrease in hepatic metabolic activity from AKI [19, 20,
27, 28]. Similarly, three animal studies of AKI on the activity of specific CYP450 enzymes did not
demonstrate any effect for the majority of CYP450 enzymes examined (CYP2A1, 2B1/2, 2C11, and
2D2) [29-31]. However, the effect on some of the CYP450 enzymes in these studies, specifically 2C6
and 3A2, appeared to depend on the animal model of AKI utilized as these enzymes showed either no
change or a reduced metabolic activity. Interspecies differences were also observed even when the
same medication probe was used. A rat study of diltiazem utilizing the uranyl nitrate model of AKI
resulted in an increase in hepatic metabolism [19]. Conversely, a rabbit study using the folate model
of AKI and diltiazem showed reduced hepatic metabolism [20]. Four human studies of AKI and
nonrenal clearance, however, appear to refute the majority of these animal study findings. A study by
Macias et al. evaluating the pharmacokinetics of vancomycin in human AKI subjects receiving
continuous venovenous hemofiltration found that nonrenal clearance appears to decrease with the

14.
duration of renal failure. Nonrenal clearance of vancomycin was initially preserved early in the course
of AKI, but eventually approached the clearance observed in patients with CKD [49]. Similar results
were also found in two other AKI studies of continuous hemofiltration and imipenem by Mueller et al.
and Vos et al [50, 51]. It is not known precisely what constitutes the nonrenal clearance in these
studies, as drug metabolism and clearance can occur in a variety of organs beside the liver. However,
if the nonrenal clearance is primarily hepatic the results are compelling; presumably the
accumulation of uremic solutes over time lead to an inhibition of the nonrenal clearance. Another
human study of AKI and hepatic metabolism by Heinemeyer et al. evaluated the pharmacokinetics of
the NSAID, metamizole and its primary metabolite monomethyl-aminoantipyrine (MMAAP) in
critically-ill patients with AKI [32]. These researchers found significantly reduced clearance of
MMAAP in patients with AKI versus normal renal function suggesting decreased hepatic metabolism.
However, they cautioned that definitive conclusions on the pharmacokinetics of metamizole and likely
other metabolized medications in AKI remain hampered by the clinical complexity and potential
confounders in the critically-ill patient. Hypoxia, decreased protein synthesis, competitive inhibition
from concomitant medications and decreased hepatic perfusion could also be explanations for the
reduced clearance of the primary metabolite in this study. In addition, though metamizole has been
shown to be an inducer of CYP450 2B6 and 3A4, and its secondary metabolite 4-
methylaminoantipyrine undergoes an enzymatic metabolism to 4-amino-antipyrine when incubated
with human liver microsomes, no specific CYP450 isoenzyme has been identified that governs the
metabolism of metamizole or its metabolites [44].
AKI and drug transport
Drug transporters are found in a variety of tissues including the liver, kidney, intestine, brain and
placenta and are classified broadly into two major classes, efflux and uptake [37]. Efflux transporters
function to excrete drugs from within cells to the extracellular space often against a concentration
gradient. Conversely, uptake transporters facilitate the translocation of drugs into cells such as
hepatocytes where metabolism can occur. Similar to the literature for CYP450 enzymatic activity,
animal studies of drug transporters in CKD have shown reduced transporter expression and activity
when compared to animals with normal renal function [41-43, 48]. The decreased transporter activity
and expression is postulated to be secondary to uremic solutes and would be expected to have a
significant clinical impact on the disposition of metabolized medications. In CKD, the two critical
drug transporters that have been studied the most are p-glycoprotein (P-gp), an efflux transporter and
organic anion transporter (OAT), an uptake transporter. Unfortunately, there are only a small number
of studies evaluating transporter function in AKI. In studies of rats with AKI, there was increased P-
gp expression in the kidney, but not in the liver or intestines [33-36]. However, despite the increased
P-gp expression in the kidney, clearance of P-gp substrates was reduced. This was also observed in
the liver and intestines leading the researchers to conclude that AKI may cause a systemic suppression
of P-gp function. In rat ischemia-perfusion models of AKI and OAT transporters, OAT1 and OAT3
mRNA and protein expression were reduced [38-40]. This resulted in a decreased renal uptake and
clearance of the OAT substrate, p-aminohippurate.
Renal replacement therapy and non-renal clearance
Research by Nolin and colleagues demonstrated that a single 4-hour session of hemodialysis increased
the nonrenal clearance of erythromycin in human subjects with ESRD [11]. This was presumably
secondary to the removal of uremic solutes that accumulate during the interdialytic period and inhibit
CYP450 3A4 and drug transporters in combination or independently. A subsequent study by Nolin et
al. of midazolam in subjects with ESRD provided further clarity and implicated transporters (hOATP
and/or intestinal P-gp) as the likely drug disposition bottle-neck in uremia rather than CYP3A4 [48].
By logical extension, one would expect a similar improvement in nonrenal clearance in AKI patients
receiving hemodialysis. However, the continuous hemofiltration research with vancomycin and
imipenem implies that this phenomenon may take time to develop as nonrenal clearance initially
remained intact in AKI in these studies [49-51]. Furthermore, despite the ongoing removal of the
inhibitory uremic solutes by the continuous hemofiltration, nonrenal clearance still declined. This
suggests that though there are many variables potentially impacting drug metabolism in a critically-ill
patient receiving continuous hemofiltration, there may be limits to the magnitude of improvement in
nonrenal clearance that can be expected from a renal replacement therapy.

15.
Drug dosing recommendations
It is apparent that drawing any clinically meaningful conclusions from the studies of AKI and non-
renal clearance applicable to drug dosing decisions in humans would be premature. Though the
majority of the animal studies of AKI showed no effect on CYP450-mediated metabolism, the animal
research on specific drug transporters did demonstrate a decrement in transporter function from AKI.
Moreover, the few human AKI studies appear congruent with the CKD literature and support a
negative effect on nonrenal clearance. There are several potential methodological issues within the
animal studies, however, that may explain the discrepancies between the animal and human data. For
one, the animal models of AKI utilized may be affecting hepatic metabolism differently. Interspecies
differences may also explain some of the disparate results. Furthermore, extrapolating the results of
the animal studies of specific hepatic CYP450 enzymes to humans must be done with a degree of
caution. Though there are general similarities, animal CYP450 enzymes are not necessarily analogous
to human CYP450 enzymes. Individual pharmacogenomic differences can also impact CYP450
enzymes conferring vastly different activities for the same isoenzyme. The effect of AKI on drug
metabolism in CYP450 activity in one organ such as the liver also cannot be reliably extrapolated to
other organs such as the intestine. Finally, other areas of uncertainty in the literature include the
human studies of critically-ill patients. The precise nature of the nonrenal clearance for vancomycin
and imipenem is unknown and may not be primarily hepatic. The metabolic pathway of metamizole is
also incompletely characterized. In addition, the inherent clinical complexity and medical regimens of
the critically-ill subjects with AKI can be profound introducing confounding variables that may have
influenced drug disposition.
Given the current uncertainty, the clinical approach to AKI patients receiving metabolized medications
should include frequent monitoring of drug response and where possible pharmacokinetic analysis of
serum concentrations of medications. Unfortunately, therapeutic drug monitoring is not always
possible in the clinical setting since serum concentrations of many of the metabolized medications are
not routinely obtained or available. The concurrent development of tests that are can provide drug
concentrations for our most critical metabolized medications in a clinically relevant time frame is
needed. In addition, the duration of AKI appears to be an important issue and should be considered
when dosing medications with significant nonrenal clearance. The practice of using the CKD and
ESRD dosing recommendations for medications with a component of nonrenal clearance may lead to
subtherapeutic levels early in the hospital course of an AKI patient when nonrenal clearance remains
largely intact. As a result, larger doses of a medication early in the course of AKI may be appropriate
with the caveat that the dose should be reduced with time as the AKI persists and nonrenal clearance
attenuates.
Future research directions
There are several potential areas for future research for AKI and non-renal clearance. To start, some
very fundamental and critical questions are still seeking answers that could help elucidate drug
disposition in AKI. These include how to accurately and reliably determine both renal and hepatic
function in AKI. In addition, given the potential confounders present in animal models of AKI and
CYP450 metabolism, more human studies are certainly needed. Further investigations of the impact
of renal replacement therapy such as hemodialysis on hepatic metabolism in the acute setting are also
necessary. The elucidation of the nonrenal clearance component of our important medications is also
warranted. For now and for unknown reasons, the balance of the emerging animal AKI literature
departs markedly from the CKD research and seems to support the conclusion that AKI does not
impact CYP450-mediated metabolism. Conversely, the impact of AKI on drug transport and the
studies of vancomycin and imipenem and nonrenal clearance agree with the CKD studies. However,
the nascent nature of this area of AKI research, the limitations of the existing animal studies and the
scant human trials make these statements less reliable and potentially ephemeral.

18.
Section 5: Drug Removal by Intermittent Renal Replacement
A. J. Atkinson and Jason G. Umans
Hemodialysis removes many small molecule drugs to a significant extent. Consequently, optimal
pharmacotherapy of patients on chronic hemodialysis and emergency hemodialysis for some patients
who have received drug overdoses are both critically dependent on the availability of reliable
information from well-designed and executed pharmacokinetic studies. From the standpoint of
pharmacokinetics, the artificial kidney is an ideal eliminating “organ” because, in contrast to renal or
hepatic routes of drug elimination, blood flow to the eliminating organ, drug concentrations in blood
entering and leaving the eliminating organ, and recovery of eliminated drug can all be measured in
routine studies (Figure 1) (1).
Figure 1. Sources of data for analysis of hemodialysis pharmacokinetics include drug concentration in blood or plasma
entering (A) and leaving (V) the dialyzer, blood flow through the dialyser (QB), and drug removal in the spent dialysate
(reproduced from reference 1).
Unfortunately, the accuracy of many studies of hemodialysis pharmacokinetics is compromised
because they do not incorporate all of these measurements in their analysis or because they use them
without consideration of additional factors such as the partitioning of drug between plasma and
erythrocytes.
Hemodialysis clearance:
Accurate estimation of hemodialysis clearance is of paramount importance in the conduct of
pharmacokinetic studies in hemodialysis patients. Elimination clearances are additive, so total drug
clearance during hemodialysis (CLE) can be expressed as the sum of a patient’s residual renal
clearance (CLR), nonrenal clearance (CLNR), and dialysis clearance (CLD):
CL E = CL R + CL NR + CL D (1)
Levy (2) proposed an arbitrary but reasonable threshold criterion by assigning significance to
dialytic drug clearance when it exceeds 30% of total non-dialytic elimination (i.e., when CLD > 30% of
CLR + CLNR). However, this comparison needs to be made using clearance estimates that are
consistent in that they are uniformly based on either plasma or blood drug concentrations. In addition
to CLD, the extent of drug removal by hemodialysis (i.e., actual mass transfer of drug from blood to
dialysate) is determined by the duration of hemodialysis sessions, the contribution of ultrafiltration
across the dialyzer, and by drug and patient factors, including drug distribution volume, binding to
plasma proteins and partitioning into erythrocytes, and dialysis-associated reductions in
intercompartmental clearance (3).
The two approaches generally used to estimate CLD are termed the recovery method and the
A-V difference method (4). The recovery method has been considered to be the “gold standard” (5).
In this method, CLD is calculated from an equation that is similar to that used to calculate renal
clearance:

19.
C D ⋅ Vol D
CL D = (2)
A⋅ t
where the amount of drug recovered by dialysis is the product of the drug concentration in dialysate
(CD) and total volume of dialysate (VolD) collected during the dialysis time (t), and A is the average
concentration of drug in plasma entering the dialyzer. The product A · t can be replaced by the area
under the afferent blood or plasma concentration curve (AUCA) during hemodialysis. Assumptions
that should be confirmed are that all drug removed as blood traverses the dialyzer actually appears in
spent dialysate and that there is no drug adsorption to the dialysis membrane as may occur with ionic
interactions, particularly with the AN69 polyacrylonitrile membrane (6). The difficulty of collecting
all the spent dialysis bath fluid during hemodialysis can be avoided by collecting timed interval
samples during the hemodialysis session. This also makes it possible to evaluate clearance changes
that might occur during the hemodialysis session
Because dialysis bath fluid is not routinely collected during hemodialysis therapy, ClD is
usually estimated using the A-V difference method which is based on the Fick Equation:
A − V
Cl D = Q B  (3)
 A  
where the terms A, V, and QB are as shown in Figure 1. The terms in brackets describe an extraction
ratio and, because plasma and blood concentrations are usually a fixed proportion of each other, either
can be used to calculate this ratio. Unfortunately, this method of estimating CLD is subject to error,
primarily because plasma flow is used inappropriately in calculating plasma clearance and because of
kinetic perturbations in drug distribution kinetics that occur during dialysis.
Whereas, it is appropriate to calculate CLD as blood clearance by setting QD equal to measured
blood flow when CLR and CLNR are also calculated as blood clearances, when CLD is most often
reported as plasma clearance and is usually estimated by setting Q equal to plasma flow (3, 4).
However, this estimate of plasma clearance is only comparable to plasma clearances calculated for
CLR and CLNR when the drug is totally excluded from erythrocytes. Otherwise, plasma clearance
needs to be estimated using the effective flow through the dialyzer (QEff), given as:
Q EFF = Q B ⋅ B / P (4)
where B and P are the respective drug concentrations in blood and plasma. For example, many drugs
partition preferentially into red blood cells and are fully or largely dialyzable from both plasma and
erythrocytes. Consequently they will have QEff values that are not less than but exceed measured
blood flow (7). Assuming that all the drug in erythrocytes is dialyzable, an appropriate B/P ratio can
be calculated from the drug’s hematocrit (H) and erythrocyte/plasma partition coefficient (RBC/P) as
follows:
B P = ( RBC P ) H + (1 − H ) (5)
However, this assumption is unnecessary when CLD is calculated by the recovery method and venous
(V) as well as arterial (A) drug concentrations are included in the pharmacokinetic analysis. In this
case, QEff can also be estimated from a rearrangement of Equation 3 and used to estimate the
dialyzability of drug that partitions into erythrocytes (7).
Kinetic perturbations during hemodialysis:
Most hemodialysis studies are conducted assuming that drug distribution and elimination
kinetics both remain unchanged during this procedure. However, Stec et al. (7) found that the
intercompartmental clearance (CLS) of N-acetylprocainamide (NAPA) between the central
intravascular compartment and the slowly equilibrating peripheral compartment of a 3-compartment
mammillary model decreased during hemodialysis to a maximum extent that averaged 77%. This
reduction in CLS was only apparent because the recovery method was used to estimate CLD and the
fall in A and V was greater than expected from the amount of drug recovered from the spent dialysate.

20.
Because the fall in arterial blood concentrations was in part due to the change in drug distribution, it
can be seen from Equation 3 that the A-V difference method would have overestimated CLD.
Of possible additional importance is that Nolin et al. (8) found that hepatic CYP3A4
metabolic activity, assessed with the erythromycin breath test, increases by 27% as soon as 2 hours
after hemodialysis, apparently due to clearance of low molecular weight uremic toxins that act to
inhibit this cytochrome. Should CLNR actually begin to increase during hemodialysis, this also would
lead to an over estimation of CLD unless it is calculated by the recovery method.
Recommendations:
Because the clinical utility of pharmacokinetic studies in hemodialysis patients is critically
dependent on the accuracy with which CLD is estimated, we recommend adoption of the recovery
method for these studies. It also is important that comparison with predialysis elimination clearance
estimates be facilitated by the appropriate selection of either blood or plasma concentrations when
calculating CLD. The drug should be administered intravenously at a sufficient interval before
hemodialysis is instituted so that predialysis distribution and elimination pharmacokinetics can be
fully characterized. Reports should include the dialyzer model and the extent of ultrafiltration needed
to compensate for interdialytic weight gain. In the ideal study, ultrafiltration would be minimized to
limit the contribution of convection to estimates of CLD. Finally, patients need to be studied long
enough following dialysis to assess the extent of the postdialysis rebound in drug concentrations.
Results of pharmacokinetic studies in hemodialysis studies are used most commonly to
estimate appropriate postdialysis supplementary doses of the drugs that are being administered to these
patients. However, emergency hemodialysis may also be life-saving for patients who have received
drug overdoses or have ingested toxic substances. In this latter setting, the beneficial effects of drug
removal by hemodialysis are augmented by dialysis-associated reductions in CLS that exaggerate the
fall in plasma drug concentrations and consequently reduce the exposure of the brain, heart, and
circulatory regulatory system to toxic drug concentrations. The fall in CLS occurs not only during the
dialysis period but is maintained for some time after hemodialysis, so has the effect of sequestering
drug in pharmacologically inert somatic tissues (7). As a result, estimates of drug distribution volume
made in these patients on the basis of the observed fall in plasma concentrations and measurements of
drug recovered during dialysis will be much smaller than those obtained in non-dialyzed subjects (9).
For this reason, hemodialysis may represent effective therapy for patients overdosed with drugs that
are freely dialyzable but currently are not considered candidates for this intervention because of their
large distribution volumes.
A continuing clinical problem is that pharmacokinetic results obtained with one dialyzer are
generally not representative of the performance of other dialyzers. Thus, there is a critical need to
characterize CLD estimates made with one dialyzer in a way that results can be readily extrapolated to
different dialyzer models. One approach could be based on in vitro studies in which the dialysis
characteristics of a standard compound are compared across different dialyzers. For example, the
contributions of dialyzer blood flow and permeability coefficient-surface area product (P·S) to CLD
were first analyzed by Renkin (10) using the following equation to calculate P·S values for a number
of solutes from values of QB and CLD obtained in vitro with a Kolff-Brigham dialyzer:
CL D = Q B (1 − e − P ⋅ S/Q ) (6)
Applying Equation 6 to results from a study in which Gibson et al. (11) compared in vitro estimates
of CLD for procainamide (PA) and N-acetylprocainamide (NAPA) for 10 different dialyzers, it was
found that the ratio of P·SPA/P·SNAPA remained relatively constant, averaging 1.28 ± 0.23, similar to the
ratio of their free water diffusion coefficients (12). For implementation of this approach, a suitable
standard compound should have a molecular weight that is larger than that of urea (MW = 60 Da) and
more typical of most dialyzable drugs so that its CLD is substantially influenced by P·S rather than
primarily determined by QB. Currently, all marketed dialyzers have clearance data for several solutes
that routinely include in vitro estimates of their mass transfer area coefficient (KoA), analogous to P·S
in Equation 6. Creatinine (MW = 113 Da), vitamin B12 (MW = 1,355 Da), and β2-microglobulin
(MW = 11,800 Da) are all commonly used to characterize currently-available dialyzers and might be
suitable standard compounds.

22.
Section 6: Drug Dosing in CRRT and Hybrid Dialysis Therapies
Bruce A. Mueller & Jan T. Kielstein
Optimal pharmacotherapy in critically ill patients with acute kidney injury (AKI) requiring renal
replacement therapy (RRT) is essential. Sepsis is the most common contributing factor to the
development of AKI,1 yet antibiotic considerations are often an afterthought in the management of
these patients. Several trials developed to determine the optimal continuous renal replacement therapy
(CRRT) or hybrid RRT intensity have failed to increase antibiotic doses to account for the increased
drug removal associated with the high-intensity CRRT arm.2,3 The higher delivered clearance was
associated with higher hypophosphatemia rates suggesting that the reported lack of superiority in
higher CRRT doses may have been due to increased drug clearance and inadequate pharmacotherapy.
These trials are an example of the larger problem: many intensivists, nephrologists and pharmacists
making dosing decisions for a particularly vulnerable patient population are unaware of how newer
forms of RRT can affect drug removal. Complicating the issue is the variability of how RRT is
delivered in intensive care units (ICUs), and the lack of generalizability of the results of existing RRT
pharmacokinetic trials.
What is known about Drug Dosing in CRRT and Hybrid RRT
CRRT is commonly used in the ICU because it provides a better tolerated form of solute removal for
hemodynamically unstable patients. However, CRRT is not a standardized therapy, and practices vary
widely. Several modes of therapy (convective, diffusive, or both), a variety of filter materials, and
different effluent flow rates are used4, all of which can influence drug removal. Despite the large
variability in CRRT techniques, a review of published dialytic clearance studies found that less than
90% of studies specified the CRRT dose employed, and only 58% of CVVH studies specified whether
pre- or post-dilution mode was used.5 Two basic pharmacokinetic values necessary for interpretation
of study results, volume of distribution (Vd) and clearance (CL), were specified in only 79% and 81%
of studies, respectively. None of the reviewed studies contained the “ideal data set” formulated by the
authors (Table 1).
Hybrid RRTs are gaining popularity in ICUs because they can be performed using standard
hemodialysis machines, can be less expensive than CRRT to perform, and require less
anticoagulation.6,7 These therapies run at higher dialysate flow rates (usually 100-300 ml/min) than
those used in CRRT, and treatment often lasts for 6-12 hours in duration, although more “continuous”
forms of hybrid therapy have been reported.8 Hybrid RRT techniques are not uniform, and many
variations exist (Table 2).7 Further complicating the issue is the lack of standard nomenclature, which
can become confusing. Hybrid therapies include slow, low-efficiency dialysis (SLED), extended daily
dialysis (EDD), continuous SLED (c-SLED), slow low-efficiency daily dialysis (SLEDD), slow low
efficiency daily hemodiafiltration (SLEDD-f), NxStage Dialysis, and others, making literature
searches and pharmacokinetic interpretation difficult. The intermittent nature of most hybrid RRTs
can further complicate drug dosing, as higher amounts of drug may be needed during the therapy,
while lower doses may be required during therapy downtime. To date, pharmacokinetic studies with
only twelve drugs have been studied with hybrid RRTs (Table 3). A variant of CRRT, high volume
hemofiltration (HVHF) has been advocated as sepsis therapy, but no pharmacokinetic studies have
ever been published in HVHF.
Despite the dearth of information on drug dosing in CRRT and hybrid RRT, the FDA and EMEA do
not currently mandate pharmacokinetic studies for these therapies. New draft FDA guidance language
published in March 2010 does not mandate these studies either9, so the lack of knowledge about
appropriate drug dosing for critically ill patients requiring these therapies is likely to persist.
When evaluating drug dosages in patients receiving RRT, patient-related, drug-related, and RRT-
related characteristics should be taken into account. Critically ill patients in AKI exhibit altered
volumes of distribution, non-renal clearance10, and serum protein concentrations compared to healthy
volunteers with end-stage renal disease, and so drug dosing in healthy patients cannot simply be
extended to include the critically ill population.11,12 Patient body weight13 and fluid status must also be
considered. Obese patients may require larger drug doses, just as severely fluid overloaded patients

23.
may require larger doses before RRT is started and fluid status is corrected. Drug-related factors
influencing RRT removal include molecular weight, degree of protein binding, and volume of
distribution. A drug is more likely to be removed by RRT if it has a smaller molecular weight,14 has a
small volume of distribution (often a cut off of <1L/kg is used), and is not substantially protein bound.
A strong correlation exists between sieving coefficient and (100% - [% Protein Binding]) for most
drugs. 15
RRT parameters substantially influence drug clearance. The mode of therapy (diffusion, convection,
or both) alone can be influential, as both therapies can remove small solutes, but convective therapies
are superior at removing larger molecular weight solutes.16,17 Drug clearance is affected by where
replacement fluids are given, because this influences the drug concentration that travels through the
filter. Mathematical calculations can account for this,18-20 but published studies do not always specify
this information.5 Filter material and type also influence drug removal. The older, cuprophane filters
did not remove larger drugs effectively in hemodialysis studies, while newer high-flux membranes can
remove substantial amounts of drug.21 Some degree of drug and other solute adsorption occurs with
many dialysis membranes (particularly sulfonated polyacrylonitrile [AN69®] and
polymethylmethacrylate [PMMA]), although it is difficult to quantify adsorption in both in vitro and
in vivo studies.22-24 Dialysis dose is one of the most influential factors of drug removal, with increased
dialysate/ultrafiltration/effluent flow rates resulting in greater drug removal (although there may be
something of a ceiling effect for diffusion-based therapies).25
Several methods have been suggested for calculating drug doses.
1) The first and most effective method would be to use therapeutic drug monitoring when
possible. However, very few drugs have clinically useful (quick turnaround time,
FDA/EMEA approved) assays available.
2) The most “evidence-based” method would be to consult current literature for studies of the
drug in RRT. Unfortunately, this literature is limited and may not be generalizable across
RRT settings and patient populations.
3) A commonly used method involves calculating a total creatinine clearance (CrCl) based on the
addition of residual renal clearance and expected extracorporeal clearance, and using
medication dosing guidelines specified for that total CrCl range.26 Using this method, most
drugs will fall in the CrCl 25-50 ml/min range. This method is useful, although it assumes
that drugs only undergo glomerular filtration, not tubular secretion or reabsorption.
4) A fourth method starts with the dose and dosing interval for a patient with a GFR < 10 ml/min
(anuric dose), and makes dosage adaptations based on the drug fraction expected to be
removed by extracorporeal therapy.27,28
i) Maintenance dose = anuric dose/ 1-FrEC
ii) Dosing interval = anuric dosing interval X (1-FrEC)
5) A fifth method starts with a normal dose (Dn), and reduces dose based on normal clearance
(Clnormal), non-renal clearance (Clnon-renal), effluent rate (Qeff), & sieving coefficient (SC).29
a. Dose CVVH = Dosen X [Clnon-renal +(Qeff X SC)]/Clnormal
All methods fail to take each drug’s pharmacodynamic profile into account. Pharmacodynamics,
which examines how drug concentrations can be optimized to meet a therapeutic goal (i.e increase
bacterial kill, reduce blood pressure, etc) is beginning to be better recognized as vital for patient
outcomes. For example, pharmacodynamic targets have been identified for many antibiotics, and
pharmacodynamic theory should be applied when making antibiotic dosing recommendations.30,31

24.
Finally, these equations may not be useful when making drug-dosing decisions with hybrid RRTs.
Although the formulas might be able to suggest a reasonable dose, further consideration needs to be
given to the timing of the drug dose in relation to the timing and duration of the hybrid RRT.
What can be achieved with current knowledge?
With the help of therapeutic drug monitoring, the current literature available, and the drug dosing
methods outlined above, appropriate dosing recommendations can be formulated for most drugs,
although confirmation with clinical trials is the only way to confirm that drug dosing is appropriate.
For patients receiving CRRT, the 2007 publication “Drug Prescribing in Renal Failure” by Aronoff
and colleagues gives recommendations for 475 drugs.32 However, only 58 of the 475 drugs have ever
been the subject of CRRT studies, and many were in vitro studies. For hybrid RRT, there is at least
some human data available for 12 drugs (Table 2). One can probably extrapolate some of this
knowledge to include other drugs (e.g. data about gentamicin dosing can probably be applied to
tobramycin dosing), but the variability in hybrid techniques makes it difficult to apply
recommendations published in one study to others.
With the state of current knowledge, efforts can be made to educate intensivists, nephrologists, and
pharmacists about the intricacies of drug dosing in patients receiving CRRT/Hybrid RRT (as with the
upcoming NEPHSAP) and the need for well-constructed drug-dosing studies that yield dosing
recommendations that can be translated to patients receiving many types of RRT.
Vision for what needs to be known to advance clinical practice
An analysis of drug dosing in the published RRT intensity trials could help elucidate whether the
reported lack of improvement in patient outcomes in the higher intensity RRT arms was due to
inadequate antibiotic dosing. In order to optimize pharmacotherapy, further pharmacokinetic studies
in RRT must be conducted. These trials should be expanded to include removal of all drugs used in
critically ill patients that do not have a reliable and readily available measurable parameter of efficacy
(e.g. blood glucose for insulin or blood pressure & heart rate for catecholamines). Pharmacokinetic
studies in children are also urgently needed. Assessment of non-renal clearance changes in AKI and
how RRT affects non-renal clearance must be performed. Finally, these pharmacokinetic studies must
be linked with pharmacodynamic measures and patient outcomes to confirm appropriate therapy
equates with improved patient outcomes.
Drug dosing recommendations on a mg/kg basis should be derived from these studies in adults and
children instead of recommending a fixed dosage for everyone.12 Special emphasis should be placed
on patients with volume overload and obesity, common conditions in the ICU, but often used as
exclusion criteria in pharmacokinetic trials.
Ideally, drug clearance studies should be done in conjunction with the development of new practices
and technologies. Citrate anticoagulation has gained popularity, but the optimal (safe and efficacious)
anticoagulation regimen for specific situations remains to be evaluated. The relationship between
hemofilter age and drug clearance should also be assessed. The influence of newer high cut-off filters,
high-volume hemofiltration, albumin dialysis, and other extracorporeal therapies (e.g. extracorporeal
membrane oxygenation or invasive lung assist membrane ventilation) on drug clearance will need to
quantified and used to make drug dosing recommendations.
New technologies that could greatly simplify drug dosing efforts include updated software that uses
drug, RRT, and patient data to generate accurate initial drug dosing regimens, and automated
pharmacy notification when CRRT goes down, perhaps with automatic dosing adjustments. The most
helpful and large-scale recommendation by far would be worldwide RRT technology and dosing
standardization in research and practice to reduce RRT variability as a source of error and increase
generalizability of this research. In this case, pharmacokinetic trials could be conducted using one
universal CRRT or hybrid RRT regimen, and results could be extrapolated widely.